Integration: fall back to the fixed radius on a pattern too dense for it

48008e144 widens the signal radius on crystals with wide spots. On one
battery crystal - simultaneously the widest-spot and among the highest in
mosaicity - the wider radius left its neighbours' background rings with too
few clean pixels and cost 28.5% of its observations. Pass 1 now measures how
often that happens and, above a bound, hands pass 2 the settings from before
the pre-scan widened them.

The obvious quantity does not work. On the total rate of reflections dropped
for a starved ring, the losing crystal reads 4.08% and the rule's four
biggest winners read 1.89-2.04% - and a crystal at the shipped radius reads
2.37%, above all of them. Re-running the winners at r1 = 4 shows why: they
read 2.20-2.32% there too, and widening moves them down. That floor is module
gaps, the beam stop and the resolution mask, which are properties of the
detector and do not move with the radius.

So the counter separates the two. A ring is neighbour-starved when it would
have kept more than five pixels but for the pixels a neighbouring
reflection's signal region occupies. That is exact rather than estimated: the
reflection mask marks the disk inside r2 and the ring is everything outside
it, so a masked ring pixel always belongs to some other reflection's core.
The separation goes from a factor of 2 to a factor of 13 - over the twelve
crystals the radius moves, the rate is 0.000 five times, 0.001 three times,
then 0.004, 0.235, 0.315 and 4.082 - and the bound is the log-space midpoint
of that one gap, 0.0113, a factor 3.6 clear of the nearest measurement on
either side.

Predicted reflection spacing does not separate them at all: the losing
crystal is 19th of 38, a winner sits at 21.9 px, and the loosest pattern in
the battery starves 1.93% of its rings.

Battery: the space group is identical on all 38 and the merged .hkl is
byte-identical on 37, so it is inert wherever it does not fire. On the one
crystal it fires on, <I/sigma> is up 18.3%, R_meas down 29.4%, observations
up 6.3%, CC1/2 0.944 to 0.974, and its two empty top shells come back as
numbers. Its indexing rate, refined distance, beam centre and cell are
bit-identical between the two arms, so this is the guard and not the two-pass
gate.

The counters are a shared channel through both engines, summed across
workers and logged once per pass; on the GPU it is one atomic add per dropped
reflection. The profile-fit runaway guard reports on the same channel, which
is the first measurement of its trip rate.

This does not recover that crystal fully. With the adaptive radius on, pass 1
reaches a different lattice and pass 2 indexes 21% fewer frames - which
happens before the measurement this guard reads exists, and is unaffected by
it. At matched indexing rate the guard recovers 96% of the baseline's
observations against 90.5% without it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CHMmeM1d489zvNFT7ZMN2P

docs/CHANGELOG.md

@@ -9,6 +9,7 @@ This is an UNSTABLE release. It includes many experimental features, as well as
 * rugnux: a lattice centring the data could not test - the crystal was integrated on the primitive sub-cell, so the reflections the centring extinguishes were never measured - is marked `UNTESTED` in the space-group candidate table and, where it is adopted, is warned about as coming from the lattice metric rather than from the intensities.
 * rugnux: the space-group search prints the twin-law disagreement H for every operator it tested and the H ratio of the point group it adopted, on every run, instead of only when that ratio refuses a promotion.
 * rugnux: `--mode scale` works on a `_process.h5` whose space group came from re-seating the lattice; a file written before this stops with a message naming the two cells instead of failing inside the merge.
+* rugnux: on a pattern too dense for the widened radius - where neighbouring reflections leave more than 1.1% of the reflections without a background ring - the second integration pass goes back to the fixed 4 px radius, and says so.
 * rugnux: on rotation data the integration signal radius is set from how wide the crystal's own spots are, measured in the pre-scan, instead of the fixed 4 px; `--adaptive-integration-radius=off` restores the fixed radius, and an explicit `--integration-radius` still overrides both.
 * rugnux: a directional diffraction limit that is the edge of the measured data rather than the crystal's own limit is marked as such - with a `<` in the report and in `ANISOTROPY_D_MIN_CENSORED`, and in the mmCIF - so `ANISOTROPY_D_MIN_SPREAD` is not read as a measurement when it is a lower bound.
 * rugnux: the anisotropy verdict line names which of `ANISOTROPY_DELTA_B` and `ANISOTROPY_DELTA_B_LINEAR` it is quoting, says which of the two to act on, and says why the second can be the larger.